The present invention relates to a method of predicting paraffin deposition risks during hydrocarbon production and/or transport, applicable at an early reservoir characterization and production stage.
Paraffins precipitation is a recurring problem in oil production. Late detection of the presence of paraffins in the oil in place in the reservoir, even in small amounts, can lead to very costly production problems (complete clogging of the production and/or transport pipes, wrong estimation of the producible reserves). In fact, during production, the cooling undergone by the hydrocarbons, either as they are expelled from the reservoir to the surface or as they are transported, leads in some cases to the crystallization of the paraffins. Similarly, degassing of the light fraction ( less than C10) of the fluid considered to be a good solvent of the high molecular weight molecules, which is linked with the pressure drop during production, favours paraffin deposition. Various known methods relative to the characterization of paraffins in production oils are described for example in the following documents:
Philp P., 1994, High temperature gas chromatography for the analysis of fossil fuels: a review, J. of High Res. Chromato., 17, 398-406,
Philp P., Bishop A. N., Del Rio J. C. and Allen J., 1995, Characterization of high molecular weight hydrocarbons ( greater than C40) in oils and reservoir rocks, The Geochemistry of Reservoirs, Geological Society Special Publication, 86, 71-75,
Heath D., Lewis C. and Rowland S., 1997, The use of high temperature gas chromatography to study the biodegradation of high molecular weight hydrocarbons, Org. Geochem., 26, 769-786,
Thanh N. X., Hsieh M. and Philp R. P., 1999, Waxes and asphaltenes in crude oils, Org. Geochem., 30, 119-132.
Patent FR-2,753,535 filed by the applicant describes a method for modelling the crystallization of paraffins in petroleum fluids according to the composition thereof, using an analytical representation of the petroleum fluids in form of pseudo-constituents in limited number comprising each certain hydrocarbon classes. From a database comprising the values of physico-chemical parameters of a certain number N of pure hydrocarbon constituents, the physico-chemical parameters of at least part of these pseudo-components are determined by combination of the corresponding physico-chemical parameters of these hydrocarbon constituents. Application of data from this base to a thermodynamic model allows to determine the wax or paraffin appearance temperatures.
Existing models work from input data obtained via analyses carried out at a late production stage, from stock-tank oils that have already lost the main part of the C40+ fraction if precipitation phenomena have already occurred in the reservoir or along production tubings. The signs that would allow a model to make an early prediction of the appearance of deposits are therefore missing from the experimental data collected.
The method according to the invention allows to predict paraffin deposition risks during production and/or transport at an early stage of characterization and production of an underground reservoir formation. It comprises forming a database characterizing the hydrocarbons in the formation, including density data acquired by well logging and data obtained by analysis of rock samples taken from the formation, an analytical representation of the hydrocarbons by a limited number of pseudo-components comprising each certain hydrocarbon classes with definition of the respective mass fractions thereof and application of data from said base to a thermodynamic model for determining one or more parameters indicative of the crystallization conditions such as, for example, the starting crystallization temperature and the solid fraction that precipitates when the temperatures falls below this critical temperature, etc.
The method comprises for example an analytical representation of the hydrocarbons in the formation by at least nine pseudo-constituents among which at least five pseudo-constituents concern the C14xe2x88x92 fraction and the rest of the pseudo-constituents concerns the C14+ fraction.
The C14xe2x88x92 fraction is for example represented by five pseudo-constituents, the first and the second (P1, P2) being representative of the n-alkanes whose number of carbon atoms is less than or equal to 14, the third one (P3) being representative of the iso-alkanes, the fourth one (P4) being representative of the naphthenes, and the fifth one (P5) being representative of the aromatics.
The C14+ fraction is for example represented by at least four pseudo-constituents, the first and the second (P6, P7) being representative of the n-alkanes whose number of carbon atoms is above 14, the third one (P8) being representative of the non n-paraffinic saturates and the fourth one (P9) being representative of the unsaturates. For the rock sample, insofar as analysis of the normal paraffins is continued up to the C70, the normal paraffins between C40 and C70 can be considered individually.
The method comprises for example, from the sample fluid analysis results, for the third and the fourth pseudo-constituents (P8, P9) comprising the heavier fractions, creating respectively two fictitious molecules defined each by a molar distribution among the various groups that constitute it and connected together by a determined mass relation, and calculating the thermodynamic properties thereof.
The paraffin deposits formed are for example likened to a non-ideal single solid phase that can be modelled by a parameter that represents the binary interactions in the solid phase.
Collection of the experimental data obtained by density logging measurements and rock sample analyses allows finer characterization of the fluids in the formation and notably of the heavy paraffin fraction ( greater than C40+) which is mainly responsible for the crystallization thereof and it allows to better constrain the thermodynamic simulation model used to predict the parameters of this phenomenon.